The proposed studies will provide an in-depth ultrastructural-biochemical examination of the process by which high density lipoproteins (HDL) deliver cholesterol to mammalian cells. Specifically, we hope to identify which cells in a given organ are responsible for HDL uptake, and which aspects of the cells themselves (surface membrane sites or intracellular organelles) are involved in HDL-uptake events. Although the majority of our structurally-oriented experiments are, of necessity, directed toward tracking the protein moiety of HDL in cells, each experiment will be linked to a parallel biochemical experiment in which the relevance of a given structural event involving HDL-protein is evaluated in terms of the cell's utilization of HDL-delivered cholesterol. In addition to these efforts, we plan to test specific morphological techniques which may allow us, in the future, to directly track cholesterol in cells. In large part, we have proposed follow-up experiments on information obtained in an earlier study using the luteinized ovary as a tissue model. The planned studies can be divided into 4 major parts as follows: 1) experiments designed to retest our observation that ovarian luteal cells in vivo do not internalize HDL-protein as part of an HDL-initiated steroidogenic response; 2) experiments designed to test the observation that ovarian cells in vivo may process HDL differently than ovarian cells in vitro; 3) experiments designed to enlarge these questions regarding HDL processing to include another steroidogenic organ (adrenal) and a non-steroidogenic organ (liver); and 4) efforts to develop morphological techniques which would permit us to track the delivery of cholesterol, itself, in tissues.